In the field of geophysical prospecting, the knowledge of the earth's subsurface structure is useful for finding and extracting valuable mineral resources such as oil and natural gas. A well-known tool of geophysical prospecting is a “seismic survey.” In a seismic survey, acoustic waves produced by one or more sources are transmitted into the earth as an acoustic signal. When the acoustic signal encounters an interface between two subsurface strata having different acoustic impedances, a portion of the acoustic signal is reflected back to the earth's surface. Sensors detect these reflected portions of the acoustic signal, and the sensors' outputs are recorded as data. Seismic data processing techniques are then applied to the collected data to estimate the subsurface structure. It should be noted that there are other geophysical survey techniques (e.g., gravimetric, magnetic, and electromagnetic surveys) that can be used to collect subsurface information and the present disclosure is also applicable to many of those survey systems.
Geophysical surveys can be performed on land or in water, and they can be repeated in order to track changes in the subsurface formations such as, e.g., reservoir depletion or movements of formation fluid interfaces. The use of such repeated surveys adds a time dimension to the data set, and accordingly it is often termed “Four-dimensional seismology” or “4D surveying”. In a typical marine survey, up to 20 streamer cables and one or more sources are towed behind a vessel. A typical streamer includes hundreds or even thousands of sensors positioned at spaced intervals along its length, which can range from 2 to 12 km. The various streamer cables are typically positioned from 25 to 150 meters apart and are preferably towed in a generally parallel relationship so as to collect survey data over a fairly uniform sampling grid.
When streamers and sources are towed through the water, they are subject to the effects of water currents that often tend to pull them from their desired paths. To combat this tendency, the streamers and sources are typically equipped with positioning devices such as those disclosed in U.S. Pat. Nos. 6,011,752; 6,144,342; 6,879,542; 6,985,403; 7,222,579; 7,423,929; 7,800,976. See also U.S. Pat. App. 2007/0247971 “Four dimensional seismic survey system and method” by inventors Semb and Karlsen. Wings and position-maintaining birds are typical examples of steering devices that are controllable to regulate the horizontal displacement of the sources or streamers relative to their desired paths. Such devices, while helpful for maintaining streamer positions, are also sources of sensor noise due to the turbulence they can generate when pushing or pulling the streamer back into position. (Such sensor noise levels can be readily measured by the system between source actuations.) Of course, the vessel itself is steerable and its path is also a factor in determining the source and streamer paths during the survey.
When conducting repeated surveys, any mismatch in the streamer paths makes it more difficult to accurately resolve differences between survey results. On the other hand, strict enforcement of streamer path matching is expected to require excessive use of streamer positioning devices, which in turn would raise the sensor noise level above the acceptable threshold.
While the invention is susceptible to various modifications, equivalents, and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto do not limit the disclosure, but on the contrary, they are examples of the modifications, equivalents, and alternatives that may fall within the scope of the appended claims.